A plate type heat exchanger is formed by a plurality of tube units and a plurality of corrugated fins. The tube unit is also formed by a couple of plates such as shown in FIG. 1. Each plate is connected to a corresponding pair of plates at an outer peripheral portion 101, a center rib 102 and at a number of ribs 103. Namely, since the brazing material of aluminum alloy is clad on both the outer surface and the inner surface of the plate 100, the brazing material clad on the inner surface of the plate 100 is gathered at the connecting portions 101, 102 and 103 in order to connect a pair of plates 100 to each other. The brazing material clad on the outer surface of the plate 100 connects the corrugated fin to the tube unit.
Since the pressurized fluid is introduced into the tube unit, the tube unit should have enough sealing efficiency. According to the present inventor's study, several tube units did not have enough sealing efficiency. Eight heat exchangers has the leaking problem where the fluid within the tube unit leaked. It has been observed by the present inventors that the tube units which had a leaking problem did not connect with each other at the center rib 102. To solve this problem the present inventors have examined the relationship between the anti-pressure strength of the tube unit and the brazing area of the center rib 102.
FIG. 2 shows the test piece of the tube unit 10. The tube unit 10 shown in FIG. 2 has unbrazed area 201 where no brazing material is supplied. FIG. 3 shows the test result used by the tube unit shown in FIG. 2 by changing the non brazing area 201. The abscissa of FIG. 3 represents the length of the unbrazed area 201, and the ordinate of FIG. 3 represents the anti-pressure strength of the tube unit 10. As shown from FIG. 3, the tube unit having much unbrazed area 201 does not have enough anti-pressure strength.
Therefore, the brazing at the center rib 102, especially at the top end 105 if the center rib 102 is strongly required in order to have enough sealing efficiency of the tube unit 10. The present inventors have studied why the unbrazed area occurred at the top end 105 of the center rib 102. According to the present inventors' study, the brazing at the center rib 102 is deemed to start from the bottom end 106 of the center rib 102 and the molten brazing material flows toward the top end 105. The amount of the brazing material at the bottom end 106 is deemed to be more than the amount of the brazing material at the top end 105. Since the bottom end 106 of the center rib 102 is integrally connected to the tank portion 110, the brazing material clad on the tank portion 101 may flow toward the bottom end 106.